Effect of strain rate on the buckling behavior of single- and double-walled carbon nanotubes

Molecular dynamics simulations are performed on single- (SWCNTs) and double-walled carbon nanotubes (DWCNTs) to investigate the effects of strain rate on their buckling behavior. The Brenner’s second-generation reactive empirical bond order and Lennard-Jones 12-6 potentials are used to describe the short range bonding and long range van der Waals atomic (vdW) interaction within the carbon nanotubes, respectively. The sensitivity of the buckling behavior with respect to the strain rate is investigated by prescribing different axial velocities to the ends of the SWCNTs and DWCNTs in the compression simulations. In addition, the effects of vdW interaction between the walls of the DWCNTs on their buckling behavior are also examined. The simulation results show that higher strain rates lead to higher buckling loads and buckling strains for both SWCNTs and DWCNTs. A distinguishing characteristic between SWCNTs and DWCNTs is that the former experiences an abrupt drop in axial load whereas the axial load in latter decreases over a finite, albeit small, range of strain after buckling initiates. The buckling capability of DWCNT is enhanced in the presence of vdW interaction. DWCNTs can sustain a higher strain before buckling than SWCNTs of similar diameter under otherwise identical conditions.     

Controllable synthesis of single- and double-walled carbon nanotubes from petroleum coke and their application to solar cells

Single- and double-walled carbon nanotubes (SWCNTs and DWCNTs) have been controllably synthesized by an arc discharge in different atmosphere using petroleum coke as carbon source. The morphology and properties of two kinds of carbon nanotubes (CNTs) synthesized with Fe as catalyst were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, UV–visible spectroscopy, inductively coupled plasma optical emission spectrometer, thermogravimetric analysis and infrared spectroscopy. In the He gas atmosphere only SWCNTs were found to be synthesized by arc discharge in contrast to the case in Ar gas atmosphere in which only DWCNTs were formed, In addition, properties of solar cells based on both kinds of CNTs and n-type Si are examined under illumination of light emission diode (LED). It is found that the performance of solar cells depends significantly on the type of CNTs, i.e., SWCNTs-based solar cells show better performance under LED illumination with wavelengths in the range of 400–940 nm than the case of DWCNTs which exhibit high performance under illumination of the 1310 nm infrared light.     

Surface-modified aluminogermanate nanotube by OPA: Synthesis and characterization

In this study, the surface of the synthesized short, single-walled aluminogermanate nanotube was modified by using an amphiphilic molecule, octadecylphosphonic acid (OPA). The synthetic products were characterized by using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and thermogravimetric analysis (TGA). The obtained evidences indicated that the alkyl chains of OPA were successfully absorbed on the surface of aluminogermanate nanotube. This modification is useful for applying the aluminogermanate nanotube to producing potential polymer nanocomposite.     

Band structure and absorption spectrum of double-walled zigzag carbon nanotubes in an electric field

The electronic structure of the (9, 0)-(18, 0) double-walled zigzag carbon nanotubes in the presence of a uniform transverse electric field is studied by the tight-binding model. The electric field could induce the semiconductor-metal transition, change the direct gap into the indirect gap, alter the subband curvatures, destroy the double degeneracy, produce the new band-edge states, make more subbands group around the Fermi level, and widen the π-band width. Such effects are directly reflected in density of states and optical excitation spectra. The absorption spectra exhibit a lot of prominent peaks, mainly owing to the rich one-dimensional energy subbands. The intensity, the number, and the frequency of absorption peaks are strongly modulated by the electric field. The modulation of electronic and optical properties is amplified by the parallel magnetic field. The predicted electronic and optical properties can be, respectively, verified by the conductance measurements and the optical spectroscopy.     

Studies on near infrared optical absorption, Raman scattering, and corresponding thermal properties of single- and double-walled carbon nanotubes for possible cancer targeting and laser-based ablation

We report the photo-thermal properties of single and double wall carbon nanotubes (CNT) dispersed in various solvents with different concentrations when exposed to near infrared nm laser irradiations. All these studies were correlated to the dispersivity of CNT in various solvents. The observed temperature increase of the various nanotube solutions was found to be determined by the concentration of the optically active nanotubes, as determined from their Raman scattering spectra, which are resonant in the spectral range of the laser excitation. Such findings could significantly improve the understanding of the optically active CNT species, their overall laser induced-heating levels, and thus reducing the amount of the CNT required for bio-medical photothermal applications to limit the possible undesired cytotoxic effects that occur at high nanotube concentrations. Our studies open up the possibility of using selective species of CNT as effective low-toxic photo thermal agents for cancer targeting and ablation.     

Nitrogen doping effects on the structure behavior and the field emission performance of double-walled carbon nanotubes

The nitrogen (N) doping effect and field emission properties of double-walled carbon nanotubes (DWCNTs) were investigated. Diameter transformation and defect generation in the N-doped DWCNTs mainly depend on the amount of nitrogen employed. By applying N-doping into DWCNTs (1.5 N at.%), the average diameters of the DWCNTs were increased from 1.7 to 2.4 nm, and the crystallinity (I_G/I_D) was decreased from 13.5 to 5. Field emission properties were enhanced by the N doping into DWCNTs. The turn-on field, corresponding to a current density of 0.1 μA/cm², was about 0.9 V/μm for the N-doped DWCNTs (1.5 N at.%). The field enhancement factor of the N-doped DWCNTs was higher than that of the undoped DWCNTs. It was found that the field emission properties were controlled by pyridine-like N in the graphite due to N-doping.     

Effect of excess silicate on the structure formation and textural properties of MTS materials

The synthesis of micellar templated silica (MTS) materials has been conducted with different SiO₂/CTAB molar ratios for two different cetyltrimethylammoniumbromide (CTAB) concentrations of 0.02 and 0.07 M to examine the structure formation for cubic (SBA-1) as well as for hexagonal (SBA-3) surfactant arrangement in acidic reaction medium with increasing silicate excess. Reaction products were analyzed with X-ray diffraction, N₂ physisorption and TG-MS. It was found that an increase of the silicate concentration leads to the formation of additional silica gel in the resulting MTS material, which causes not only a decrease of specific surface area and pore volume, but also of the mesopore diameter. The latter effect is explained by a proposed embedding theory. Furthermore, with increasing silicate content in the synthesis mixture a phase transformation from hexagonal to cubic pore arrangement was found for the CTAB concentration of 0.07 M and the maximum SiO₂/CTAB molar ratio for ordered pore arrangement has been determined.     

Connection of macro-sized double-walled carbon nanotube strands by bandaging with double-walled carbon nanotube films

Based on their special shrinkage characteristics, double-walled carbon nanotube (DWCNT) films were used as bandages to bind the overlapped ends of macro-sized (centimeters long) DWCNT strands for connection to get structures of random length. Tensile tests indicated that the joints made in this way had relatively high tensile strength with a maximum value of 311 MPa corresponding to that of the original strands. The equivalent contact resistance of the joints was very small. And the connected strands showed better electronic properties in our investigation on the temperature dependence of resistivity and the same remarkable current capacity, in contrast to the original ones. This technique may offer a promising potential for the future extensive use of macro-sized CNTs in many fields, such as electrical cables and wires.     

Torsional buckling of double-walled carbon nanotubes

The mechanical instability of doubled-walled carbon nanotubes subject to torsion motion is investigated through molecular dynamics. A newly revealed buckling mode with one or three thin, local rims on the outer tube was discovered while the inner tube shows a helically aligned buckling mode in three dimensions. The distinct buckling modes of the two tubes imply the inapplicability of continuum mechanics modeling in which it is postulated that the buckling modes of the constituent tubes have the same shape. In view of this problem, a new concept of the equivalent thickness of double-walled carbon nanotubes is introduced, which enables the Kromm shell model to be applied to the derivation of the torsional buckling angle without the restraint of the two tubes having identical shapes.     

Torsional elastic instability of double-walled carbon nanotubes

In this paper, a theoretical analysis of the torsional buckling instability of double-walled carbon nanotubes (DWCNTs) and the DWCNTs embedded in an elastic medium is presented based on the continuum elastic shell model and Winkler spring model. Using the proposed theoretical approach, the influences of the aspect ratio, the buckling modes and the surrounding medium on the torsional stability are examined in detail. The simulation results show that the torsional instability of DWCNTs can occur in different buckling modes according to the aspect ratio. The van der Waals (vdW) interaction force between nanotubes reinforces the stiffness of nanoshells. Thus, the DWCNTs possess higher buckling stability than the SWCNTs without considering vdW interaction force.     

Electronic and optical properties of double-walled armchair carbon nanotubes

Magnetoelectronic structures of double-walled armchair carbon nanotubes are calculated according to the tight-binding model. Their features are dominated by the intertube interactions, the symmetric configurations, the magnetic flux, and the Zeeman splitting. The drastic changes of the low energy states, such as energy dispersion, wave function, and Fermi level, which also rely on the different symmetries, are caused by the intertube interactions. The magnetic flux could change linear bands into parabolic bands, destroy state degeneracy, open an energy gap, and shift Fermi level. The magnetic flux and the intertube interactions, however, compete with each other in the metallic or semiconducting behavior. The Zeeman splitting would suppress the metal–semiconductor transition while the opposite is true of the magnetic flux. The main characteristics of energy bands are directly reflected in the magneto-optical absorption spectra. The different symmetric configurations can be distinguished by the absorption peaks, and the threshold absorption frequency is not identical with the energy gap.     

Electronic properties of double-walled carbon nanotube films

The electronic properties of as-prepared and purified DWNT films were tested using the four-probe method. The resistivity of the purified DWNT films is about 1 mΩ cm at room temperature and has a positive dρ/dT above 55 K, which shows a good metallic property. The electrical resistivities of the purified DWNT films are quite similar to those of the SWNT bundles and acid-treated SWNTs. Our results are correspondent with the early theoretical calculation on the band structure of DWNTs.     

Effect of flow field heterogeneity in coagulators on aggregate size and structure

Aggregate size and structure were investigated under turbulent conditions in stirred tank (ST) and Taylor–Couette‐type (TC‐type) devices. Root‐mean‐square radius of gyration, 〈Rg〉, and zero‐angle intensity of scattered light, I(0), were acquired as a function of stirring intensity, characterized by an experimentally obtained average hydrodynamic stress, 〈τ〉exp, determined by torque measurements. Evaluating aggregate images revealed that aggregate structure and shape are independent of the device type. However, in TC‐type devices, the aggregates grow to three to four times larger sizes than inside ST, although the same 〈τ〉exp was used in both coagulators. As confirmed by computational fluid dynamics, this can be attributed to the differences in the maximum hydrodynamic stress in ST compared with those in TC‐type devices. In contrast, the power‐law scaling of 〈Rg〉 and I(0) with 〈τ〉exp is preserved for all investigated devices, with an exponent approximately equal to ?0.5 and ?0.7, respectively. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Transport properties of double-walled carbon nanotubes and carbon boronitride heteronanotubes

Using the density functional theory combined with the nonequilibrium Green's function, the transport properties of double-walled carbon nanotubes (DWCNTs) and carbon boronitride (CBN) heteronanotubes were investigated. As the hopping length increases, the conductance of DWCNTs shows a dramatic variation that is independent of the intertube space. The transport of the CBN heterojunctions also displays abnormal behavior when the hopping length is changed, which is very different from the behavior of DWCNTs. The currents of the forward in the CBN heterojunctions are about 3–15 times as large as those of the back under lower bias voltages. The negative differential resistance (NDR) effect occurs in the CBN heterojunctions, and the peak-to-valley ratio in the additional NDR regions is about 2–4 for the current–voltage relationship. The hopping length and BN parts have a great influence on the transport of the double-walled nanodevices.     

Thermal and textural characteristics of modified silica

A silica gel characterised as predominently microporous, with the remaining pores of limited dimensions which do not permit capillary condensation, is modified with aqueous solutions of acetic acid, glycine and β-alanine. The modified samples are investigated and compared with a water soaked sample by means of differential thermal analysis, thermogravimetry and i.r. spectroscopy. Textural measurements are also carried out on the parent samples and those produced in the temperature range 110–600°C. Acetic acid is both physically and chemically adsorbed; a surface ester is formed. Both amino acids are physically adsorbed but no chemical interaction has been detected. Glycine and β-alanine decompose differently when adsorbed on to the silica surface than when in the pure state. The specific surface area decreases upon modification, reaching half the value of the pure silica for the β-alanine modified sample. Pore structure analysis, shows that the microporous nature of the modified samples decreases in the modifier order acetic acid < glycine < β-alanine. The adsorbed β-alanine molecules block most of the micropores present.     

The characterization of interfaces between textural components in metallurgical cokes

Six coals, representing the rank range normally encountered in commercial coking, were carbonized in a small oven to give dense cokes, of tensile strength comparable with that of good-quality blast-furnace coke. Interfaces between the different textural components in the cokes were studied by polarized-light microscopy. It proved possible to classify interfaces according to their perceived quality, to quantify their occurrence by point-counting and to calculate interface quality indices for the coke as a whole or for interfaces involving individual textural components. Interfaces between vitrinite-derived reactive coke components were superior to those involving inerts, but the inerts content of a coke did not have a marked influence on the coke interface quality index. The highest coke interface quality index was observed for the coke from the coal with the highest dilatation. No clear evidence of an influence of interface quality on coke tensile strength is apparent from the present data.     

Tensile properties of ultrathin double-walled carbon nanotube membranes

Direct tensile tests of double walled carbon nanotube (DWCNT) membranes with thickness of 40–80 nm were performed using a micro-stress-strain puller. The tensile strength and Young’s modulus are 4.8E2–8.4E2 MPa and 4.4–8.8 GPa, respectively. The deformation and fracture processes were analyzed using the stress vs. strain curves, and SEM observations of the fracture surface of a membrane. The membrane experienced elastic strain and plastic strain during tensile-loading to fracture, and the plastic process is due to the real plastic deformation of the membrane and the slippage between the DWCNT bundles. Cracks occur and spread during the tensile test which causes the membrane to be mangled. With these excellent mechanical properties, the DWCNT membranes can be used in nanotube-reinforced composites.